A tale of 3 empires

Here is a comparison of the words "America", "England" and "China" as used in English-speaking books throughout the 20th century. It shows how England's influence waned, America's ascended (especially during the war years) and how China's has remained fairly constant since the 1940s.

Notice that searches on the Ngram Viewer are case sensitive. I originally (mistakenly) compared "china", "america" and "england" (all lower-case) and of course, "china" was at the top by a significant margin, since this refers to the pottery.

Come caveats

While these searches are really interesting, we need to ask:

Which books are included, and which are not included yet?

Which books are not included due to copyright issues?

The data doesn't appear that useful for the naughties (2000 to 2010). Many terms seem to decrease in importance, or increase inexplicably.